Many industries utilize highly viscous paste which must be highly pressurized for filtering and packaging of the paste. Such industries include the electronics, pharmaceutical and chemical industries. For instance, specifically, in the electronics industry, thick film conductive paste is used in multi-layer ceramic packaging of semiconductor devices. Typically, such paste can have a viscosity in the range of 25,000 to 75,000 centipoise (CPS) or greater, and a pressure in the range of 300 to 1,500 pounds per square inch (PSI) or greater may be required to properly move the paste through, for example, a wire mesh filter with 40 micron openings.
Previous paste pressurizing apparatus utilized a high pressure pneumatic extrusion pump fed by a low pressure pneumatic ram follower plate system. However, such pressurizing apparatus generally resulted in unpredictable and imprecise paste pressures which reduced filter life and made pressure measurements difficult and impractical. Moreover, the follower plate system generally introduced air into the high pressure extrusion pump, causing the undesirable result of air being incorporated into the paste product. These extrusion pump systems also tended to add frictional impact to the product due to the required pumping action. Frictional impact causes particle deformation and leads to the undesirable result of metallic platelet formation in the product, thus reducing product quality.
Another disadvantage associated with extrusion pump systems is that considerable amounts of residual paste remain in the system after processing is complete. This is due to the overall design of extrusion pump systems which does not allow for complete discharge of product from the system. Incomplete paste discharge is wasteful in terms of cost and efficiency. Further, since extrusion pump systems incorporate many components assembled in a complicated manner, residual paste is a particularly significant problem when the same pump system is required to pump more than one type of paste, and the different paste types cannot be intermixed. In this regard, it is extremely difficult and time-consuming to disassemble the pump system for cleaning residual paste from the various components before pumping another paste type.
Generally speaking, U.S. Pat. No. 4,819,836 to Meckenstock discloses a dispenser for paste compositions. However, the Meckenstock dispenser is manufactured by injection molding and is thus designed only for the dispensing of relatively low viscosity paste compositions, such as toothpaste. The Meckenstock dispenser design cannot be employed in high pressure applications for moving high viscosity pastes. Furthermore, the problem of introduction of air into the product persists in the Meckenstock dispenser.
U.S. Pat. No. 4,951,848 to Keller discloses a viscous material dispenser with a vented delivery piston for avoiding the inclusion of air in the storage cylinder above the viscous material contained in the dispensing cartridge. The piston has a vent hole passing therethrough which seats a closing screw. However, the closing screw must be manually tightened for closing the vent hole after all air is released. Thus, the air release mechanism of the Keller dispenser is inefficient and tedious. Moreover, the overall design of the Keller dispenser is incapable of pressurizing highly viscous fluid to high pressures.
Thus, there remains a need for a viscous fluid pressurizing apparatus which is capable of exerting high pressure on highly viscous fluid, which substantially discharges the entire amount of product contained in the apparatus, which is easily assembled and disassembled, and which automatically and conveniently releases air before pressurizing so that air will not be included in the product as a result of the pressurizing apparatus.